Problem: The scale on a map is 8cm : 4km. If two cities are 80cm apart on the map, what is the actual distance in km between the two cities?
The scale means that every 8cm on the map corresponds to an actual distance of 4km. A distance of 80cm on the map is the same as 10 $\cdot$ 8cm. The actual distance between the two cities is 10 $\cdot$ 4km, or 40km.